Is the game played on just 1 map?
Is there any progression to be had? As all levels reset for each new game.
Is there anything which you keep as an overall aim/goal?
Yeah it is but valve plans to add mod tools eventually I believe which can be really good fun.
You get the basic level up depending on how much you play. Everytime you do level up you are guaranteed an item, all items are purely cosmetic but some awesome ones, as far as progression that is it though which is a good sign in some ways as too many games need unlocks to persuade people to play these days.
You may be interested in item trading I don't know, some of the stuff you can get from chests that drop (require a £2 key to open I think but you can trade items for them) can have very rare couriers worth in excess of £100, even around £300.
Also you can get strange cosmetics that record kills, wards bought etc and things specific to each class